Catalytic converters are commonly used with conventional internal combustion engines to reduce undesirable constituents in the exhaust gas thereof. The catalyst of these converters is destroyed by unburned fuel passing to, and combusting in a hot converter. Converter life expectancy and efficiency thus deteriorates when the engine passes unburned fuel through to the catalyst, such as when an ignition event fails to completely burn the fuel charge in a cylinder due to an engine misfire. Furthermore, as a catalytic converter converts engine emissions most efficiently when a stoichiometric mixture is properly combusted in the engine, misfires can reduce converter conversion efficiency and increase vehicle emissions.
It is therefore desirable to accurately detect and categorize engine misfires so that those misfires that contribute significantly to increased vehicle emissions or to potential catalytic converter damage may be corrected. It has previously been proposed to detect engine misfire by analyzing engine pressure or by monitoring the engine exhaust gas mixture. Furthermore, because misfire reduces the work output of the engine, thus causing a short period of deceleration after which the engine, through the work contribution of other combustion events, will accelerate to pre-misfire speeds, it has been proposed to compare engine speed in one cylinder firing event to a firing event in another cylinder, so as to provide a relative measure of work output.
Still further, it is known to use an engine speed sensor, such as a position encoder, to monitor the relative average speed between consecutive cylinder events, for the purpose of measuring this relative work. Such position encoders typically take the form of a wheel, with a number of teeth disposed at substantially equidistant positions about its circumference. The wheel is disposed on the engine such that it rotates at an angular velocity proportional to the angular velocity of the engine output shaft.
The difficulty with detecting misfires according to the relative work product of consecutive cylinder combustion events is that, especially at high engine speeds, engine speed is more sensitive to disturbances in the sense that a single engine speed disturbance, such as a misfire, can result in significant deceleration over more than one cylinder event. Additionally, engine speed disturbances can affect engine speed in much the same way as a typical misfire. Furthermore, at high speeds, even slight variations in the distance between teeth on the position encoding wheel can produce significant measurement error which may reduce the integrity of the misfire detecting system.